The realization during the past decade that optical fibers may play a significant and commercially viable role in communication systems--both because of their increased information carrying capacity and their decreased size--has led to a dramatic increase in research and development into various fiber structures, as well as into more efficient and cost effective fabrication processes.
There are currently two major processes which dominate the fiber fabrication field. The first, disclosed in part U.S. Pat. No. Re. 28,029, generally involves the formation of glass particulate material by flame hydrolysis and its subsequent deposition on a solid glass rod. After an appropriate thickness of glass particulate material has been deposited on the rod, the rod may be removed, and the material is consolidated into a transparent glass by heating in an appropriate environment. The resultant tubular "optical fiber preform" is then drawn into a fiber, preceded by or simultaneous with, collapse of the tubular preform. The glass precursor particulate material used in this process is generally formed from volatilized glass precursor liquids (e.g., silicon tetrachloride, germanium tetrachloride and boron trichloride) using a hydrolysis burner. In view of the fact that this particulate material is formed in a flame, it is commonly referred to as "soot"--to be distinguished from glass precursor particulate material formed using other processes and without the use of a hydrolysis flame burner.
The second prevalent fiber fabrication process--the Modified Chemical Vapor Deposition process (MCVD)--involves the reaction of appropriate vapor species located within a glass tube, to produce glass precursor particulate material (see U.S. patent application Ser. No. 828,617, filed Aug. 29, 1977, a continuation of application Ser. No. 444,705, filed Feb. 22, 1974. The glass precursor vapors are flowed through the tube while the tube is heated with an appropriate heat source. The particulate material formed in this reaction subsequently deposits on the interior of the glass tube. After a sufficient deposit has formed, the glass tube is collapsed and drawn into a fiber. The deposited particulate material comprises the core, and in certain embodiments the cladding, of the optical fiber, while the tubular starting member generally comprises an appropriate jacket.
The MCVD technique may be traced historically to the prevalent semiconductor technology. This technology requires the formation of ultrapure silicon oxide layers. It was found that such layers may be efficiently grown by heating an appropriate substrate in a gaseous environment of silicon-containing-vapor and oxygen. Silicon oxide was found to deposit heterogeneously on the surface without the formation of a particulate phase. In fact, the formation of such a particulate phase was in many circumstances deleterious and was specifically avoided.
In a significant departure from this prior art semiconductor teaching, the MCVD fiber fabrication process involves the formation of silicon oxide by means of an intermediate particulate phase. Such a particulate phase is found to result in significantly increased deposition rates without sacrificing the required purity of the deposited material.
The MCVD process must be distinguished from the previously discussed hydrolysis process in which the particulate material is formed in a flame--and consequently referred to as "soot." The soot process has its historical roots in previous hydrolysis technology. Unlike the users of MCVD, the practitioner of the soot process must consider the significant impurity problems primarily due to the formation of water vapor--a strong absorber of light in the spectral range of interest--during the hydrolysis reaction.
The modified chemical deposition process has met with great success. However, it presents an apparently academic question of interest to basic researchers in the field--namely, what is the mechanism responsible for the deposition of the particulate material on the tubular wall. Gravitational forces apparently are not adequate to explain this phenomenon. However, in view of the fact that the process works effectively, this question was relegated to the sphere of unanswered academic problems whose solution would have little commercial ramifications.